Here we have a classic example of arguing Ad Hominem, a tactic often used within the realm of politics, but by no means limited to it. When arguing Ad Hominem (which literally means “to the man”) the idea is to undermine your opponent's position by attacking the character of those who espouse it, rather than addressing the merits of the position itself.
The intention of the I Once Saw A Vegan Wearing Leather Shoes! argument is to cast doubt upon the entire premise of veganism by attempting to expose the hypocrisy or inconsistency (apparent or actual) of at least one vegan. This is of course ridiculous. The arguments for veganism stand or fall on their own merits and are neither supported nor undermined by the personal habits or integrity of those who embrace, or purport to embrace, the philosophy.
This argument is even more tenuous in light of the fact that many convincing, cruelty-free alternatives to leather now exist and even from a close distance, they are all but indistinguishable from the (so to speak) genuine article. How anyone could make a cursory examination of someone else's footwear and determine conclusively the material out of which it was made is something of a mystery, but this is nonetheless the claim. As explained above, however, even if the observation were accurate, it would say nothing at all about the case for animal rights, animal liberation, and veganism.
Your argument is invalid.